It is our intent to continue our comprehensive evaluation of the organization of the inferior olivary nucleus by examining its neonatal development in the American opossum. These developmental studies are designed to describe the temporal changes within the inferior olive from birth, 132 days after conception, until weaning 90 days. Particular attention will be directed toward synaptogenesis as related to the timed arrival of midbrain, cerebellar and spinal afferent systems, as well as to the development of olive-cerebellar projections. Our focus on the olive is based on its probable role in both cerebellar development and motor learning; our selection of the opossum as the experimental model is related to the unique opportunity offered by the species for early manipulation in development, enabling us to define the ontogeny of known afferent and efferent connections.